Rf sputtering apparatus including a capacitive lead-in for an rf potential



Get. 17, 1967 LAEGRED ETAL 3,347,?7

RP SPUTTERING APPARATUS INCLUDING ACAPACITIVE LEAD-IN FOR AN RFPOTENTIAL Filed March 2, 1964 7'0 a S SUPP! y 19 SUPP-1 INVENTORSI WiWUnited States Patent 3,347,772 RF SPUTTERING APPARATUS INCLUD- ING ACAPACETIVE LEAD-IN FDR AN RF PDTENTIAL Nils Laegreid, Minneapolis,Minn.,

son, Rochester, N.Y., assignors to G. T. Company, a corporation ofMinnesota Filed Mar. 2, 1964, Ser. No. 352,415 (Filed under Rule 47(a)and 35 U.S.C. 116) 5 Claims. (Cl. 204-298) The present invention relatesgenerally to a material deposition system, and more particularly to afilm deposition system wherein one or more specific materials are beingdeposited upon the surface of a substrate member. The present inventionis particularly concerned with a low pressure supported gas dischargeplasma or sputtering system wherein a RF field is employed to establishand maintain a negative sputtering potential on an electrode or severalelectrode elements in a low pressure supported gas discharge plasmazone.

In a deposition system utilizing a gaseous discharge plasma phenomena, acathode and an anode are arranged in spaced apart relationship across acertain gap zone. The gap zone or area is provided with molecules oratoms of an inert gas, these molecules being subjected to electronbombardment in the gap zone whenever the cathode and anode arerespectively energized. The term molecules when used in reference to theinert gas employed in the operation is meant to include molecules in theconventional sense as well as atoms or groups of atoms of the inert gas.Collisions occur and the gas becomes partially ionized, forming chargedparticles. In the gap zone, a target or source is arranged which isadapted to have its surface bombarded by these ionized inert gas atomsand molecules, the impact energy of the ionized gas atom or moleculebeing adequate to remove surface atoms from the source material. Themetallic atom is thereafter free to move through the system andultimately a certain number of these atoms are condensed or otherwisecollected on the surface of a substrate element. Sputtering techniquesare particularly adaptable for use in connection witha wide variety ofmaterials including metals, nonmetals, compounds, refractory materialsincluding beryllium oxide, and the like. It has beenfound thatsputtering techniques are particularly advautagous in connection withmaterials which are otherwise difiicult to work with when usingconventional vacuum deposition techniques, electron bombardmenttechniques, and the like. The control available from a sputteringoperation is furthermore far superior to that control which is availablein connection with conventional techniques such as vacuum deposition andthe like. In accordance with the present invention, an improvement isavailable for sputtering operations wherein a RF field is establishedacross certain of the electrodes such as, for example, the target (i.e.source) electrodes, the RF field generating means including a capacitorwhich has a dielectric material interposed therebetween. Sputteringtechniques are particularly valuable for use in connection with thedeposition of films of metals, non-metals, compounds such as refractorycompositions and the like. The technique is particularly valuable forits versatility and'unusual control. It has been further found that asuperimposed RF field may be advantageously applied to two or moreelectrodes, such as, for example, the anode and one of the otherelectrodes, or between a pair of targets of similar or dissimilarcomposition, or between the anode and the substrate. The use of thisfield applied to these electrodes contributes to the enhancement ofproperties of materials which are prepared in accorda 'ce wtih the useof sputtering techniques. Because of the n ture of the sputteringoperation, this feature is particularly valu and Roger M. Mose-Schjeldahl 3,347,772 Patented Oct. 17, 196'? able to the operation ofthe equipment. In addition, any means by which added versatility can beacquired such as by establishing a RF field at various of the electrodesis likewise considered valuable. In accordance with the presentinvention, resonant circuit is utilized in connection with the RF field,and the capacitive portion of the RF circuit is obtained by utilizingthe capacitive effect of the cross-sectional thickness of the bell-jarenvelope which is utilized in establishing the enclosure for thesputtering system. The present invention permits a direct routing of theRF energy to the point of desirable exposure within the enclosure bymeans of the utilization of a coaxial line which shields the linecarrying the RF energy to the conductive plate backing the materialwhich is being sputtered. Thus, the application of RF energy to thetarget location or source electrode is specific, definite, andcontrollable. The bell-jar envelope is, generally, fabricated fromglass, the thickness being about one-half inch to three-fourths inch inthe normal glass bell-jar enclosure. More specifically, sides of theglass envelope and the glass therebetween serves as the dielectric forthe system. While separate electrodes are normally utilized, it is, ofcourse, possible to employ conductive paint on the outer surface of thejar for the outer electrode, if desired. While the inner electrode maybe so fabricated, the particular material forming combinations of sourceand substrate may prevent the use of this technique on the interior. Ofcourse, balancing means are required between the various portions of thecapacitive circuit, such as, for example, a tuning capacitor arranged onopposite sides of the inductively coupled circuit. These bell-jar maynot be precisely the same, from one point to another, as well as otherparameters in the system may also be changed.

Therefore, it is an object of the present invention to provide animproved technique for use in connection with a sputtering depositionoperation wherein the technique employs a superimposed RF field acrosselectrodes or Working stations, the glass enclosure It is still afurther object of the present invention to provide an improved couplingtechnique for superimposing a RF field across certain of the workingstations of the RF generator being coupled to a resonant LC circuit, themajor portion of the capacitive component of the LC circuit beingobtained by utilizing the transverse thickness of a glass bell-jar andthe transverse thickeners of the source material as the dielectric of acapacitor in the circuit.

Other and further objects of the present invention will become apparentto those skilled in the art upon a study of the following specification,appended claims and accompanying drawings, wherein:

FIGURE 1 is a perspective diagrammatic view of a sputtering impositionsystem embodying the aspects of the present invention, the figureshowing a schematic illustration of typical circuitry which may beemployed in connection with the operation of the equipment illustratedin this figure; and,

FIGURE 2 is a detail view, partially broken away, showing the capacitivecoupling feature utilized in connection with the present invention.

In accordance with the preferred modification of the present invention,the deposition system generally designated 10 includes the bell-jar 11situated upon a working base or support 12. The bell-jar 11 and thesurface 12 define an enclosure or cavity 13 which provides the a pair ofplates are disposed on opposite present invention. A conduit 14 is incommunication with the enclosure 13, and preferably is with a vacuumpump or the like which is referred to by the character 15. The pump maybe conveniently utilized to evacuate the enclosure 13, whenever thisoperation is indicated. A gas supply system is also utilized inconnection with the bell-jar enclosure, a conduit 17 being incommunication with the enclosure 13 through the base 12, and being inoperative relationship with a gas regulating valve 18 for controllingthe rate of flow of gas from a gas supply such as is designated at 19.Various inert gases may be utilized for either assisting in flushing theenclosure, or for actual use in connection therewith, or both. Forexample, argon gas is particularly desirable for use in low pressuresupported gas discharge plasma deposition operation. For a generaldescription of deposition techniques, such as those techniques which arecommonly referred to as sputtering techniques today, reference is madeto that certain article published in the Physical Review, vol. 102, No.3, pp. 69()'70'4.

The electron generating tube generally designated 21 is shown mounted independing sealed relationship with the base 12, the tube 21 also beingin direct communication with the enclosure chamber 13. The tube 21includes a thermal emission cathode or filament element 22 which obtainselectrical energy from the conductors 23 and 24, these conductors beingoperatively associated with a suitable power supply designated at 27.Since the requirement of the power supply 27 is merely to providecurrent to heat the cathode, the voltage amplitude is not critical, andis determined by the actual requirements of the cathode filament 22. Theanode electrode 28 which is fabricated from titanium or the like issupported in the chamber 13 by means of the conductive support rod 29,rod 29 being secured to and insulated from the base 12. In order to beable to both support and electrically bias the anode 28, conductor 29and the arm 30 are preferably fabricated from electrically conductivematerial and are insulated from the base 12. The post 29 is, of course,insulatively joined to the base plate 12.

An electrode member is disposed in the plasma zone established withinthe chamber 13- between the cathode 22 and the anode 28, the electrode35 being supported on the base 12 by the rod 52 and the bracket 34, thisbracket preferably being fabricated from an insulator material orotherwise electrically isolated from rod 52. Conductor 37 is coupled tothe electrode 35 through the bell-jar capacitor and the coaxialconductor 38 disposed within the rod 52. A unidirectional power sourceor supply 39 is available in order to provide sufficient DC energy toinitiate and maintain the gaseous plasma between the cathode and theanode. Resistor 40 is utilized to control the potential available onthese electrodes. Conductor 41 couples one side of the RF power supply44 to the anode element 28. A substrate or work piece 46 is disposedwithin the chamber 13 and is supported onthe base plate 12 by means ofthe bracket 47. The substrate 46 is designed for receiving a depositionsuch as a film deposit or the like along its surface 48. The substratemay be electrically biased, if desired; however for normal sputteringoperations this is not required, the substrate being at a floatingpotential.

Attention is now directed to the circuitry diagram indicated atFIGURE 1. In this regard, a suitable RF source 44 is coupled in an LCcircuit as indicated. The main source of the inductive component. isfound in the transformer generally designated 50 which includes theprimary winding 51 and a secondary 53, these windings being coupledtogether by an air medium. A suitable tuning capacitor such as a bladecapacitor or the like is shown at 54 for the purpose to be hereinaftermore specifically pointed out. The main source of the capacitivecomponent of the LC circuit is obtained across the belljar 11 and thesource 35, A pair of conductive plates operatively associated flow beingparallel of the 515 and 56 are disposed on opposite sides of bell-jar11, p ate sion element or conductor 52. The conductor 52 is thereaftercoupled electrically to the target element 35. The provision of thecoaxial extension permits the RF field to be applied specifically to atarget or source disposed within a certain specific area of the bell-jarenclosure. Sputtering will accordingly be confined to the desiredsurface area of the source, such as the source 35. Capacitor 58 isinterposed at the secondary of the LC circuit, this capacitor beingutilized as a balancing capacitor for the system. In actual practice,the primary of the trans.- former 50 includes four turns of a copperconductor about a one-quarter inch copper tube, the secondary being oneturn of copper conductor about a similar quarter inch copper tube. Thetwo windings are spaced apart by a one-half inch air gap.

The RF source 44 is preferably capable of delivering an output in therange of from one megacycle up to megacycles, and preferably in therange of about 30 megacycles. It is felt that theetfect of the RF fieldin the system may diminish below about one megacycle.

The advantage and improvement is the ability to locate the source in adesirable location in the plasma within the enclosure at a desired areain the plasma zone. In accordance with the technique of the presentinvention, the RF field is introduced into the chamber by means of acoupling technique which is disposedat a substantial distance from theremaining active stations; hence the effect of this field at the surfaceof the source is predictable.

While the specific embodiment herein disclosed illustrates a RF fieldestablished between the anode electrode and the target electrode, itwill be appreciated that other combinations are equally appropriate. Inthis connection, a RF field may conveniently be established between apair of dissimilar target elements, the effect being to deposit mixturesof materials on a substrate surface. For example, if a deposit is beingprepared which is desirably a mixture of two individual components, a RFfield may be established at these individual targets and the coatingprepared on the substrate will be a composite of the individualingredients. For example, it may be desirable to sputter a mixture ofglass and copper onto a substrate. In this connection, one target may befabricated from copper metal while the other may be prepared from aglass surface such as Pyrex glass bonded to a suitable conductive mediumsuch as copper or the like. In this con-. nection, however, the glasssurface of the Pyrex glasscopper laminate is the one which is seen ormade available by exposure to the substrate element.

Example I For a specific illustration of a deposition procedure, thefollowing example is provided. The chamber 13 wasevacuated by the vacuumpump 15 until a residual pressure of 10 torr was achieved. The valve 18was opened and a continuous supply of argon gas was admitted, the pump15, maintaining in the enclosure a pressure of 10- torr. A target wasprepared utilizing a Pyrex coated copper metal, this Pyrex materialbeing available on the surface of the target 35. Pyrex glass,one-eighthinch thick, was coated over the copper metal. The source of RFenergy delivered 2500 volts at a frequency of 27.12 megacycles to aprimary winding consisting of four turns of a copper conductor on aone-quarter inch copper tube, the secondary being one turn of copperconductor on a one-quarter inch copper tube, the primary and secondarybeing separated by a one-half inch air gap. The surface of the targetwas spaced from the surface of the substrate by a distance of fourinches, this gap distance being in the plasma generation area whichexists between the cathode 22 and the anode 28. The Pyrex bell-jar had anominal thickness of five-eighths inch, and two one inch by one inchelectrodes were arranged on opposite sides thereof.

55 being electrically coupled to the coaxial exten-.

The cathode is then energized with the element being heated to atemperature which is sufficient to cause thermal emission of electronstherefrom. While the thermally emitted electrons flow toward the anode,which is charged by the power supply 39 to a potential of 45 volts forinitiating and maintaining the plasma across a four inch gap, and whilethese electrons are passing through the gas which is present in theenclosure or chamber, collisions will occur with the atoms and moleculesof the gas, the frequency of these collisions being determined, ofcourse, by the mean-free-path existing in the atmosphere between thecathode and the anode. When these collisions occur, electrons will bedislodged from the atom or molecule of the gas and a positively chargedgas particle will be thereby formed. These charged particles areattracted to the target or source electrode 35, and the collisions whichoccur between the charged gas atoms or molecules or particles and thesurface of the target 35 cause a dislodging of material from the surfaceof the target 35, these materials then being free to move toward thesubstrate 46, and a statistical quantity of the dislodged atoms willlodge on the surface of the substrate. This portion of the operation is,of course, typical and conventional in the sputtering techniques. Inthis operation, the RF potential of 2500 volts was applied between thesurface of the target 35 and the potential of the plasma. The substratewas maintained at a floating potential being isolated from the base 12.Accordingly, the gaseous plasma will be attracted to the negativelycharged target 35.

The control available by the sputtering technique is obviouslydesirable, and with the added feature of a RF field available, thechoice of source materials is expanded to include insulators.

While the example specifically utilizes a deposition of Pyrex from abase of copper, it will be appreciated that other materials may besuitably sputtered by this technique, as wel as mixtures of materials,multiple sources of materials and the like.

In another operation, a RF field was impressed between the target andthe base 12, With suitable potentials being established on the anode,such as a potential of 45 volts, other operations being similar.

It Will be appreciated, of course, that the specific examples givenherein are for purposes of illustration only and are not to be otherwiseconstrued as a limitation upon the scope to which the present inventionis entitled. Therefore, those skilled in the art may depart from thespecific examples without actually departing from the spirit and scopeof the present invention.

What is claimed is:

1. In a deposition system utilizing a low pressure supported gasdischarge plasma for maintaining an enclosure fabricated at least inpart from a dielectric material, an inert gaseous atmosphere of lowpressure maintained within said enclosure, a plurality of workingstations disposed within said enclosure, and means for establishing agas discharge plasma therewithin, said Working stations including anelectron emitting electrode, a plurality of electrodes including ananode for said electron source disposed in spaced relationship from saidelectron emitting electrode and biased electrically to attract saidelectrons and defining a reaction zone for said electrons and the gasmolecules comprising said atmosphere, and at least one target electrodedisposed in said reaction zone, certain of said gaseous molecules beingadapted to bombard the surface of said target to remove atoms therefrom,means for collecting said removed atoms, and field generating means forestablishing a RF field generating means including capacitive meanshaving a pair of electrically coupled electrode plates disposed inspaced apart relationship on opposite surfaces of the dielectric of saidenclosure and including an inner and an outer plate, and coupling meansfor locating and isolating said RF potential on the target surfacedisposed within said enclosure and in spaced relationship from saidenclosure surface, said coupling means including an electrical conductorextending from said inner plate to said target and a shield disposedabout said conductor and generally coextensive With said conductor toelectrically isolate the field created along the extent of saidconductor from said plasma.

2. The deposition system as defined in claim 1 wherein said dielectricis glass.

3. The deposition system as defined in claim 1 being particularlycharacterized in that said RF field is established from a resonant LCcircuit, and said shield is disposed generally coaxially with saidelectrical conductor and forms a complete enclosure thereabout.

4. In a deposition system utilizing a low pressure supported gasdischarge plasma, an enclosure fabricated at least in part from adielectric material for maintaining an inert gaseous atmosphere of lowpressure maintained within said enclosure, a plurality of workingstations disposed within said enclosure, and means for establishing agas discharge plasma therewithin, said working stations including anelectron emitting electrode, a plurality of electrodes including ananode for said electrons disposed in spaced relationship from saidelectron emitting electrode and biased electrically to attract saidelectrons, and defining a reaction zone for said electrons and said gasmolecules, and at least one target electrode disposed in said reactionzone, certain of said gaseous molecules being adapted to bombard thesurface of said target to remove atoms therefrom, means for collectingsaid removed atoms, and field generating means for establishing a RFpotential on the surfaces of at least two of said electrodes, said fieldgenerating means including a reson ant LC circuit having a primaryWinding and a secondary winding, and capacitor means coupled to saidsecondary winding, said capacitor means including a pair of electricallycoupled electrode plates disposed in spaced apart relationship onopposite surfaces of said dielectric of said enclosure, and couplingmeans for locating and isolating said RF potential on a surface disposedwithin said enclosure and in spaced relationship from said enclosuresurface, said coupling means including an electrical conductor extendingfrom said inner plate to said target and a shield disposed about saidconductor and generally coextensive with said conductor to electricallyisolate the field created along the extent of said conductor from saidplasma.

5. The deposition system as defined in claim 4 being particularlycharacterized in that said coupling means includes a pair of conductorsdisposed in coaxially arranged relationship, the outer conductor beingmaintained at ground potential, and completely enclosing the innerconductor.

References Cited UNITED STATES PATENTS 1,715,952 6/1929 Rostron 343-8502,049,677 8/ 1936 Urtel 331-167 2,164,595 7/1939 Siebertz 2041923,233,137 2/ 1966 Anderson et al 313-201 OTHER REFERENCES Wehner:Advances in Electronics and Electron Physics, vol. VII, 1955, pp.246-254.

JOHN H. MACK, Primary Examiner. R, K- M HAL s st n Exam ner

1. IN A DEPOSITION SYSTEM UTILIZING A LOW PRESSURE SUPPORTED GASDISCHARGE PLASMA FOR MAINTAINING AN ENCLOSURE FABRICATED AT LEAST INPART FROM A DIELECTRIC MATERIAL, AN INERT GASEOUS ATMOSPHERE OF LOWPRESSURE MAINTAINED WITHIN SAID ENCLOSURE, A PLURALITY OF WORKINGSTATIONS DISPOSED WITHIN SAID ENCLOSURE, AND MEANS FOR ESTABLISHING AGAS DISCHARGE PLASMA THEREWITHIN, SAID WORKING A GAS DISCHARGE PLASMATHEREWITHIN, SAID WORKING STATIONS INCLUDING AN ELECTRON EMITTINGELECTRODE, A PLURALITY OF ELECTRODES INCLUDING AN ANODE FOR SAIDELECTRON SOURCE DISPOSED IN SPACED RELATIONSHIP FROM SAID ELECTRONEMITTING ELECTRODE AND BIASED ELECTRICALLY TO ATTRACT SAID ELECTRONS ANDDEFINING A REACTION ZONE FOR SAID ELECTRONS AND THE GAS MOLECULESCOMPRISING SAID ATMOSPHERE, AND AT LEAST ONE TARGET ELECTRODE DISPOSEDIN SAID REACTION ZONE, CERTAIN OF SAID GASEOUS MOLECULES BEING ADAPTEDTO BOMBARD THE SURFACE OF SAID TARGET TO REMOVE ATOMS THEREFROM, MEANSFOR COLLECTING SAID REMOVED ATOMS, AND FIELD GENERATING MEANS FORESTABLISHING A RF FIELD GENERATING MEANS INCLUDING CAPACITIVE MEANSHAVING A PAIR OF ELECTRICALLY COUPLED ELECTRODE PLATES DISPOSED INSPACED APART RELATIONSHIP ON OPPOSITE SURFACES OF THE DIELECTRIC OF SAIDENCLOSURE AND INCLUDING AN INNER AND AN OUTER PLATE, AND COUPLING MEANSFOR LOCATING AND ISOLATING SAID RF POTENTIAL ON THE TARGET SURFACEDISPOSED WITHIN SAID ENCLOSURE AND IN SPACED RELATIONSHIP FROM SAIDENCLOSURE SURFACE, SAID COUPLING MEANS INCLUDING AN ELECTRICAL CONDUCTOREXTENDING FROM SAID INNER PLATE TO SAID TARGET AND A SHIELD DISPOSEDABOUT SAID CONDUCTOR AND GENERALLY COEXTENSIVE WITH SAID CONDUCTOR TOELECTRICALLY ISOLATE THE FIELD CREATED ALONG THE EXTENT OF SAIDCONDUCTOR FROM SAID PLASMA.